Ciencia

CARBOHYDRATE FRACTIONS OF LEMON PEEL1
WALTON
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IN C LAI R

A ND

PA U

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C R A N DA L L

Received January 10, 1949.

Introduction The peel of citrus fruits is physiologically important in fruit growth and maturity in that the vascular system is confined chiefly to the mesocarp (albedo), which thus serves the important function of transporting water and solutes fromthe trees to the juice vesicles of the pulp. To understand the principles of the biochemical mechanisms occurring in the fruit during growth and development, one has to know the chemical composition of the peel and the relation between the constituents. Such data are also essential for the understanding of certain physiological relations between peel and pulp, and for the understanding of themetabolic reactions of the peel under storage conditions. The phase of the work which has to do with the organic acids and buffer properties of citrus peels has already been published (21). The present studies have been concerned with the carbohydrate fractions of lemon fruit peel. Samples of peel from lemons have been thoroughly extracted with 80 per cent. ethyl alcohol and separated intoalcohol-soluble and alcohol-insoluble fractions. The alcohol-soluble fraction contains chiefly the mono- and di-saccharides; it also contains other substances such as essential oils and waxes, and various concentrations of undetermined substances. The alcohol-insoluble fraction contains the cell-wall constituents, such as cellulose, lignin, pectin, and hemicellulose (sometimes designated as polysaccharidesor acid-hydrolyzable material). These substances, particularly pectin and hemicellulose, appear to be closely related. The pectins are not usually considered as part of the carbohydrate reserve of the plant. The readily available carbohydrate fractions serving as sources of energy consist of the total-soluble-sugars and the starch-dextrin fractions. The carbohydrate material of the cell walls isnot necessarily composed of condensation products of the simple sugars, but may consist of uronic acids united with the hexose and pentose sugars. Pectins belong to this general class of compounds, and any hemicellulose fraction that may accompany the pectins on weak-acid extraction of the tissues probably contains a uronic acid group. Cellulose, hemicellulose, and lignin compose the structural,and not the energy-yielding, units of the tissues; consequently, they are relatively inert to the metabolic activity of the cells.
Materials and methods Alcohol-insoluble solids and alcohol-soluble solids were determined on
1 Paper no. 601, University of California Citrus Experiment Station, Riverside, California. 681

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PLANT PHYSIOLOGY

the peel from 50 to 100 lemon fruits havingdiameters of approximately 21 inches. For moisture determinations, disks of peel were removed from the equatorial region of the fruit with a cork borer (no. 9). Other disks were taken from the peel, weighed, and used for the quantitative determination of the alcohol-insoluble and alcohol-soluble solids. A much larger sample of the remaining peel was used for obtaining a large experimental sample ofalcohol-insoluble material. For quantitative determinations on the small disk samples, the peel was ground in a Waring blendor in the presence of 95 per cent. alcohol, heated to boiling and allowed to stand overnight in 300 ml. of 80 per cent. alcohol, a correction being made for the moisture in the sample. Two additional extractions were made with hot 80 per cent. ethyl alcohol, and for eachextraction the samples remained in the extractant for 24 hours. The insoluble material was filtered and washed, first with 95 per cent. ethyl alcohol and. finally with petroleum ether. The colorless product was dried to constant weight at 650 C and weighed quantitatively. The percentages of alcoholinsoluble solids were calculated from this weight. Pectin was extracted from the alcohol-insoluble solids...